A polymercaptan composition

ABSTRACT

A polymercaptan epoxy resin hardener composition and a process for curing epoxy resins through the use of said composition wherein the curing process yields an epoxy adhesive possessing excellent high temperature stability.

United States Patent [191 Doss Assignee:

A POLYMERCAPTAN COMPOSITION Richard C. Doss, Bartlesville, Okla.

Phillips Petroleum Company, Bartlesville, Okla.

Filed: Sept. 1, 1970 Appl. No.: 68,770

Inventor:

us. Cl. 260/455 A, 260/2 N, 260/47 EN, 260/59 1111.01. C01c 155/08 Fieldof Search 260/455 A, 47 EN.

References Cited UNITED STATES PATENTS "2/1968 De A6605 260/609 R 7/1968Eating et al....L 260/609 R [111 3,742,006 June .26, 1973 I FOREIGNPATENTS OR APPLICATIONS 8,262 3/1968 Japan 260/455 A 507,899 11/1967France 260/455 A 701,712 12/1953 Great Britain .1 260/455 A PrimaryExdminereLewis' Gotts Assistant Examiner-G. Hol lrah Attorney -Young andQuigg 5 7] ABSTRACT l Claini, No Drawings A POLYMERCAPTAN COMPOSITIONThis invention relates to novel polymercaptan compounds, their method ofpreparation, and to the use thereof in curing epoxy resins. In anotheraspect, this invention relates to a process for curing epoxy resins toimpart exceptional high temperature utility through the application ofthe novel polymercaptan epoxy resin hardener herein disclosed.

Epoxy adhesives are basically thermosetting materials made by reactingepoxy resins with curing or hardening agents. The epoxy group is a3-membered ring,

which is under great strain and will open easily through chemical means.A variety of compounds possessing active hydrogen atoms, for example,amines, acids, phenols, alcohols and thiols, are capable of opening thering and forming a large molecule which in turn reacts with anotherepoxide starting a repetitive process which lasts until a hard andfusable material forms. These compounds containing active hydrogen atomsare called curing or hardening agents. The hard and fusable materialsare termed cured or hardened. In general, these cured products arecharacterized by good mechanical and high temperature strength, goodheat and chemical resistance, and by outstanding ability to adhere to awide range ofsubstrates. The term epoxy resin is here defined to meanthe monomer or prepolymer bearing epoxide end groups, wherein saidepoxide end groups are to be subsequently involved in reactions with theactive hydrogen-bearing curing agent. The term epoxy adhesive applies tothe cured polymer. Many different types of epoxy resins are availableand can be used in the practice of this invention. Three resin typeswhich are commonly important are listed as follows:

a. Diglycidyl ether of bisphenol A resin (generally called bisphenolAepichlorohydrin resins, which are the most widely used epoxies).

b. Epoxy-novolacs (more accurately, .glycidylated novolac resins).

c. Cycloaliphatic epoxies. The polymercaptan epoxy hardener of thisdisclosure is applicable to the various types of epoxy resins andparticularly to the three aforementioned classes of epoxies. Epoxyadhesives are well known for the wide range of substrates to which theyare applicable. Metals, wood, concrete, polyolefins and essentially anysolid substrate are included. The identity of the curing agent stronglyinfluences both the conditions requisite for curing and also theproperties of the resulting epoxy adhesive. Mercaptan or thiol curingagents possess an important advantage in their wide range of acceptablecuring temperatures, for example, from below room temperature to thecustomarily required elevated temperatures. Their reactivity at roomtemperature permits utilization with unusually low-temperature sensitivesubstrates. Additionally, mercaptan or thiol curing agents areconveniently fast acting. Moreover, the mercaptan-cured epoxy adhesivespossess superior bonding strength around room temperature and below. Atelevated temperature, however, the mercaptan-cured epoxy adhesives areusually inferior in strength, for example, to amine-cured epoxyadhesives.

It is therefore an object of this invention to provide a polymer-captancuring agent for epoxy resins and a method for preparing same. It is afurther object of this invention to provide a process for curing epoxyresins with said polymercaptan compounds wherein the re sulting epoxyadhesives will retain their bonding properties at high temperatures.

The substance of my invention involves the preparation of apolymercaptan curing agent through the reaction of certain otherpolymercaptan compounds with selected diisocyanates. These resultingpolymercaptan curing agents can be utilized for the hardening of epoxyresins whereby the epoxy resins are cured or hardened to an infusableand insoluble product possessing high temperature adhesive strength andperformance.

The novel epoxy hardener of my invention can be synthesized from thereaction of 2,4-tolylene diisocyanate with 1,2,3-propanetrithiolaccording to the following equation:

This reaction can also be utilized in preparing the other polymercaptanepoxy curing compounds of my inven-' tion.

In synthesizing the polymercaptan epoxy curing compounds of the instantinvention, the mole ratio of trithiol to diisocyanate should be 2:1 orgreater. Suitable polymercaptan reagents for the preparation of theepoxy hardening polymercaptan of my invention, are as follows:1,2,3-propanetrithiol, 1,2,6-hexanetrithiol, 1,2,4-butanetrithiol,l,3,5-cyclohexanetrithiol, tolylene-2,5-dithiol, and the like. Suitablediisocyanates which can be reacted to produce the curing agent of myinvention are as follows: 2,4-tolylene diisocyanate, 2,6- tolylenediisocyanate, 1,5-naphthalene diisocyanate, p-phenylene diisocyanate,4,4-diphenylmethane diisocyanate, durene diisocyanate, and the like.

Epoxy resins having high temperature stability can be cured by theprocess of my invention through the use of the above curing agent. Theprocess for curing epoxy resins according to my invention provides apolymercaptan hardener to epoxy resin ratio by equivalent weight of fromabout 1:075 to 1:1.25.

The epoxy resin curing agent of my invention is synthesized atatmospheric pressure in a three-stage process. In the first stage,2,4-to1ylene disiocyanate is added to 1,2,3-propanetrithiol at 0-40C(preferably 25C) over a 0.75-1.5 hour period (preferably 0.75 hour). Thereaction is completed by continued stirring at 0-40C (preferably 25C)for 05-24 hours (prefer.- ably 1.0 hour) followed by further continuedstirring at 25-l00C (preferably 45C) for 0.5-2 hours (preferably 1.0hour).

In synthesizing the novel epoxy hardener of the instant invention, forexample, 2,4-tolylene di(2,3- dimercaptopropylthiolocarbamate), the moleratio of trithiol to tolylene diisocyanate should be at least 2:1 orgreater, but no advantage is gained in employing excess trithiol beyondthat necessary to insure complete utilization of the diisocyanate. Whilea trithiol to diisocyanate ratio of 2.0 is stoichiometric, a ratio ashigh as 3.0 may be used, with a ratio in the range of 2.0-2.5 beingpreferred. The trithiol-diisocyanate reaction can occur conveniently inany non-reactive, polar solvent which can be subsequently readilyexcluded from the product upon completion of the reaction, for example,acetone, methyl ethyl ketone, methyl isobutyl ketone, ether, dioxane,tetrahydrofuran and the like. The concentration of thistrithiol-diisocyanate reaction mixture must be sufficiently dilute topermit ample opportunity for complete utilization of all isocynategroups, thus preventing significant polymer formation. Broadly, aconcentration range comprising -50 per cent reactants can be used, witha range of 25-35 percent being preferred.

The process for synthesizing the polymercaptan epoxy resin hardeningagent of my invention can be best illustrated by means of the followingexample.

EXAMPLE I Synthesis of 2,4-TolyleneDi(2,3-dimercaptopropyithiolocarbamate) 2,4-Tolylene diisocyanate (35g., 0.2 mol) in anhydrous methyl ethyl ketone (100 ml.) was addeddropwise over a 45-minute period, under a nitrogen atmosphere at 25C to1,2,3-propanetrithio1 (56 .1 g., 0.4 mol) in anhydrous methyl ethylketone (150 ml.). The mixture was stirred for an additional hour at 25C,followed by one hour at 45C. After cooling, the volatiles were removedat reduced pressure to afford a 91 g. yield of 2,4-to1ylenedi(2,3-dimercaptopropylthiolocarbamate). Analysis: Calculated for C H NO S SH, 29.1; Found: 30.4.

As indicated hereinabove, the polymercaptan of my invention is useful inthe curing of epoxy resins. The epoxy adhesives cured by the use of theepoxy hardener of the instant invention possess remarkably hightemperature uitlity. The epoxy adhesive derives from, for example, Epon828 (2,2-bis(4-[1-oxa-3,4-epoxybutyl]- phenyl)propane; epoxy equivalentweight, 185-192) hardened by the polymercaptan of the instant inventionpossesses a demonstrated utility useful over the range l8C to 125C. Theupper useful temperature range for these polymercaptan hardened epoxyadhesives will be 150C or higher. The polymercaptan hardened epoxyresins of my invention possess high temperature utility while alsopossessing low temperature adhering properties. Even though the uppertemperatures as demonstrated by my invention can be found in the art,

for example, amine hardened resins, such resins require high temperaturecuring. The polymercaptan hardeners of my invention produce resins whichcan be cured at room temperature or below, and yet possess a usefultemperature range of up to 150C or higher. Clearly, an epoxy adhesivestable at these upper temperature ranges which can be cured at roomtemperatures or below, fulfills a need found in the art.

The process of curing epoxy resins with the polymercaptan hardenerhereinabove disclosed can be best illustrated by means of the followingExample II. Example [1 demonstrates theability of the epoxy hardener ofthe instant invention, 2,4-toly1ene di( 2 ,3dimercaptopropyltiolocarbamate), to generate epoxy adhesives of superiorhigh temperature performance.

EXAMPLE I! 2,4-To1ylene Di( 2,3-dimercaptopropylthiolocarbamate) as anEpoxy l-lardener Epon 828 (19.0 g.) and 2,4-tolylene di(2,3-dimercaptopropylthiolocarbamate) (10.5 g.) were combined and applied toaluminum strips for lap shear strength testing. The polymer system washardened at 25C/24 hours followed by final hardening at 100c/24 hours.The lap shear strength test was made in accordance with the standardtest procedure ASTM designation: D l0O2/53T. Lap shear test data thusobtained appear as the first line in the Table I.

Tabulated below in Table I are comparative data showing the superiorhigh temperature performance of the tetramercaptan hardener of theinstant invention employing Epon 828 as the epoxy compound.

TABLE 1 Adhesive Lap Shear Strength, psi

(aluminum-aluminum) Hardener 25C 50C C C 1 2 5C invention 2626 3020 35902740 993 Propane trithiol 2956 2927 1307 267

